feat: create util stack consolidating utility services

Consolidates 6 utility services into a single util stack:
- drop (PairDrop) - P2P file sharing
- joplin (Joplin Server) - Note-taking and sync
- links (Linkwarden) - Bookmark manager with Meilisearch
- mattermost (Mattermost) - Team collaboration
- vault (Vaultwarden) - Password manager
- tandoor (Tandoor) - Recipe manager

Changes:
- Created util/compose.yaml with all 6 services
- Moved rtc_config.json to util directory
- Updated environment variables to use UTIL_ prefix
- Updated restic volume references to util_* names
- Container names changed from {service}_app to util_{service}
- Volume names changed from {service}_* to util_{service}_*
- Removed individual stack directories (drop, joplin, links, mattermost, vault, tandoor)
- Updated compose.yaml to include util/compose.yaml instead of individual stacks

All services maintain their original hostnames and functionality.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

util/rtc_config.json

@@ -0,0 +1,20 @@
+{
+  "sdpSemantics": "unified-plan",
+  "iceServers": [
+    {
+      "urls": "stun:stun.l.google.com:19302"
+    },
+    {
+      "urls": "stun:stun1.l.google.com:19302"
+    },
+    {
+      "urls": "stun:stun2.l.google.com:19302"
+    },
+    {
+      "urls": "stun:stun3.l.google.com:19302"
+    },
+    {
+      "urls": "stun:stun4.l.google.com:19302"
+    }
+  ]
+}